Method for making a section of a support strut member of an offshore oil-drilling rig

ABSTRACT

The invention concerns a method for making a section of a support strut member of an offshore oil-drilling rig comprising steps which consist in: a) machining each longitudinal edge of a semi-cylindrical stiffener ( 13 ) to provide along the longitudinal edge, a lip ( 18 ) for support on a main surface ( 12 A) of a main plate ( 12 ), by producing along the longitudinal edge an outer bevel ( 20 ) on the side opposite the conduit ( 17 ) defined by the stiffener ( 13 ); b) applying a base plate ( 22 ) of the support lip ( 18 ) to weld it with main surface ( 12 A) of the plate ( 12 ); heating the support lip ( 18 ) to weld it with the plate ( 12 ) and constitute a support weld joint; and d) forming a weld bead with filler metal from outside the conduit ( 17 ) in the space delimited between the support weld joint, the outer bevel ( 20 ) and the main surface ( 12 A) of the main plate ( 12 ).

The present invention concerns a fabrication process for an offshore oilproduction platform support leg member.

Oil platforms and jack-up platforms, in particular, feature legs thatbear on the seabed and a hull mounted such that it can move and beadjusted in height on the support legs.

The whole platform is floated out to the drilling or production site andthe legs are lowered into contact with the seabed then, by bearing onthe legs, the hull is raised above sea level up to an elevation thatplaces it outside the range of the highest waves.

Each support leg is made up of vertical members, for example three,interconnected by a lattice of metal struts.

Each member is made up of sections welded end to end, each formed, onthe one hand, by a rectangular main plate and, on the other hand, bysemicylindrical stiffeners in the shape of half-shells, each welded ontoone of the main surfaces of said main plate.

Generally, the length of each section is between 12 and 35 meters.

The lateral edges of the main plates incorporate teeth, which, over partof the length of the members, form diametrically opposite racks designedto operate jointly with the output gears of drive mechanisms mounted onthe platform hull.

The length of each stiffener welded onto one of the main surfaces of therectangular plates is shorter than the length of a plate to provide anopen space at each end of the plate to allow sections to be welded endto end, when the member is assembled.

To ensure stiffener continuity, a connection piece in the form of ahalf-shell with a profile corresponding to the stiffener profile isinserted at each open space in extension to the stiffeners, afterwelding the member sections end to end.

This arrangement allows previously shop-fabricated sections of themember to be joined end to end and welded directly on site.

A process for fabricating the members and, in particular, a process forwelding the stiffeners onto the main plates of the members is describedin document FR-2 719 611.

The members, which support the platform hull, are subjected to highfatigue stresses, and the weld beads, in particular those joining thestiffeners to the main plates, are also highly stressed.

The object of the invention is to propose a fabrication process, whichenables the structural and corrosion fatigue strengths of thelongitudinal welds to be increased at the lowest possible cost.

To this end, the subject of the invention is a fabrication process for asection of an offshore oil production platform support leg member, themember comprising a main plate incorporating, on at least onelongitudinal edge, teeth forming a rack and at least one stiffener ofsemicylindrical shape welded onto a main surface of the main plate alongtwo longitudinal edges, the stiffener and the plate delimiting aconduit, characterized in that it includes the following stages appliedto at least one part of the length of the member:

a) machining of each longitudinal edge of the stiffener to form, alongthe longitudinal edge, a lip for bearing against the main surface of themain plate by forming, along the longitudinal edge, an external bevel onthe side opposite the conduit;

b) applying a sole of the bearing lip to the main surface of the plate;

c) heating the bearing lip to ensure its welding to the plate and toform a bearing weld; and

d) with filler metal, forming a weld bead from outside the conduitwithin the space defined between the bearing weld, the external beveland the main surface of the main plate.

Depending on particular implementation methods, the process has one orseveral of the following features:

-   -   heat supply to cause heating of the bearing lip for the purpose        of welding it to the plate is ensured from inside the conduit;    -   heating of the bearing lip to ensure its welding to the plate is        performed without introducing filler metal;    -   heating of the bearing lip to ensure its welding to the plate is        performed using an inert gas nonconsumable-electrode arc welding        method;    -   it includes a stage, applied to at least one part of the length        of the member, involving machining of each longitudinal edge of        the stiffener to form an internal bevel on this edge, on the        side of the bearing lip facing the conduit;    -   the weld bead formed from outside the conduit fills completely        the space defined between the main surface of the plate, the        external bevel and the bearing lip;    -   the bearing lip is incorporated within the half of the stiffener        thickness situated on the internal conduit side;    -   the distance separating the stiffener internal lateral surface,        defined on the conduit side, from the bearing lip is between 20        and 40% of the stiffener thickness;    -   the bearing lip features an internal lateral projection on the        conduit side near to its sole;    -   the minimum height of the projection is between 0 and 8 mm;    -   a hollow profile is defined between the projection and the        internal bevel;    -   the height of the bearing lip measured at the base of the        external bevel is between 6 and 12 mm; and    -   the width of the sole is between 2 and 15 mm.

The invention will be better understood on reading the followingdescription, which is provided only as an example and made in referenceto the appended drawings, in which:

-   -   FIG. 1 is a diagrammatic elevation view of an oil platform, for        example a jack-up platform;    -   FIG. 2 is a diagrammatic perspective view of a section of a        support leg member fabricated by the process according to the        invention;    -   FIG. 3 is an exploded sectional view of a main plate and two        stiffeners before assembling these components;    -   FIG. 4 is a larger scale, diagrammatic sectional view showing a        connection zone between a main plate and a stiffener before        welding;

FIG. 5 is a diagrammatic sectional view of the main plate and stiffenersclamped before welding;

-   -   FIGS. 6 and 7 are larger scale, sectional views of the        connection zone between a main plate and a stiffener during the        initial welding phase; and    -   FIG. 8 is a larger scale, sectional view of a connection zone        between a rectangular plate and a stiffener after implementing        the process according to the invention.

A jack-up oil platform featuring a hull 1 mounted such that it can moveon the vertical legs 2 designed to bear on the seabed 3, when theplatform is in a drilling or production position, is diagrammaticallyrepresented in FIG. 1.

In the present case, each vertical leg 2 is of triangular sectioncomprising three vertical, parallel members 10 interconnected by alattice of metal struts 4.

The bottom of each leg 2 features a foot 5.

Moreover, at each leg 2, the platform is equipped with a drive andsuspension mechanism 6 for the hull with respect to said legs 2.

Drive mechanisms allow the legs 2 to be lowered into contact with theseabed then, by bearing on the legs, the hull 1 can be raised above sealevel up to an elevation that places it outside the range of the highestwaves.

Each member 10 of the legs is made up of several sections 11 of greatlength: of the order of 12 to 35 meters. Such a section is representedin FIG. 2.

Each section 11 is formed by a rectangular main plate 12 and twostiffeners 13 in the shape of half-shells, each welded onto one mainsurface 12A of said rectangular plate 12.

The lateral edges of the rectangular plate 12 incorporate teeth 14 toform, on the member 10, two diametrically opposite racks designed tooperate jointly with the output gears of the drive mechanisms 6, whichare not represented.

The thickness of the main plate 12 is between 120 and 220 mm.

The assembly of a section 11 of member 10 using the process according tothe invention will now be described in reference to FIG. 3 andsubsequent figures.

The stiffeners 13 are of semicylindrical shape and more specifically inthe shape of half-cylinders in the envisaged method of implementation.The thickness e of each stiffener 13 is between 40 and 120 mm.

The stiffeners 13 are welded onto the main plate 12 along thelongitudinal edges 16 running parallel to each other.

The conduit 17 is defined between the plate 12 and each stiffener 13.

As illustrated in FIG. 4, the longitudinal edges 16 of the stiffenersare prepared by machining to give a profile suitable for welding.

In particular, each longitudinal edge 16 is machined to provide a lip 18for bearing on a main surface 12A of the plate. An external bevel 20 isformed along the edge 16 on the side opposite the conduit 17, betweenthe lip 18 and the external surface 13B of the stiffener.

Advantageously, the lip 18 extends along the full length of the edge 16.This lip 18 defines a flat sole 22 bearing on the main surface 12A ofthe plate. This sole is machined such that it lies perfectly flatagainst the main surface 12A during assembly of the stiffener and theplate.

Advantageously, the bearing lip 18 is incorporated within the half ofthe stiffener 13 thickness e situated on the internal lateral surfaceside of this stiffener.

More precisely, the distance d separating the stiffener internal generalsurface 13A from the bearing lip 18 is between 0 and 50% of thethickness e of the stiffener 13, as illustrated in FIG. 4.

Preferably, this distance d is between 20 and 40% of the thickness e ofthe stiffener 13 and is preferably more or less equal to one third ofthis thickness.

Within the stiffener 13 projected thickness, offsetting of the bearinglip 18 away from the internal lateral surface 13A is achieved throughincorporation of a bevel 24, formed along the edge 13 on the conduit 17side. This internal bevel 24 is generally straight and defines, with thesole 18 or the main surface 12A, an angle α between 40° and 70° andpreferably more or less equal to 55°. On the external side, the bevel 20formed along the edge 16 defines, with the sole 22 or the plate mainsurface 12A, an angle β between 20° and 40° and preferably more or lessequal to 30°.

The bearing lip 18 is by and large formed at the point of convergence 0of the two bevels 20 and 24. With respect to these bevels, it projectsin the direction defined by the involute φ of the stiffener 13 thatpasses through the point 0.

More precisely, in section and on the external bevel 20 side, thebearing lip 18 has an external face 26 that extends generallyperpendicular to the sole 22 and connects one end of the sole 22 to thebevel 20. The height h of the face 26 is between 6 and 12 mm.

On the conduit 17 side, the bearing lip 18 is tangentially connected tothe bevel 24 at a connecting point A. The sole 22 extends beyond theprojection of point A on the sole 22 or the main surface 12A to form aninternal lateral projection 28.

The denoted minimum height T of this internal lateral projection 28 isbetween 0 and 8 mm. This projection is connected to the bevel 24 by aconcave section 30 forming a channel of semi-circular cross section.

The presence of the internal lateral projection 28 ensures that the sole22 is offset towards the conduit 17 side with respect to the stiffenerinvolute φ.

In particular, the bearing lip 18 has a general cross section whoseshape flares gradually towards the sole 22 on the internal side of theconduit 17.

The width 1 of the sole 22 is between 2 and 15 mm. This width is dividedinto a width t, corresponding to the distance separating the face 26from the point of intersection of the internal bevel 24 and the sole 22,and a width P, corresponding to the distance separating the point ofintersection of the internal bevel 24 and the sole 22 from the internalend of the sole.

The width t is preferably between 2 and 5 mm, while the width P isbetween 0 and 10 mm.

To undertake actual connection of a stiffener edge 16 and the main plate12, the stiffeners are first applied to the two opposite main surfaces12A of the main plate 12. They are held in position by temporary flanges40, which clamp the stiffeners 13 against the main surfaces of the plate12. These flanges are made up of steel sections defining generally aforked shape. These flanges are arranged at regular intervals dependingon the length of the member section and are interspaced by a distance ofbetween 3 and 5 meters.

These flanges are dimensioned such that the bottom section denoted 42and the lateral sections denoted 44 bear against the mid-section of eachstiffener 13 and the flanks, in the immediate vicinity of the edges 16to be welded, respectively. The sides of each flange feature brackets46, capable of pressing against the edges of the main plate 12 in thearea where teeth 14 are formed. The bolts 48 are inserted in thecorresponding holes drilled through the brackets 46, ensuring connectionof the two flanges each located on one side of the plate 12. The boltshanks extend between the teeth 14.

Clamping of the flanges by the bolts inserted between the teeth 14ensures that the stiffeners 13 are held in position prior to welding.

The edge 16 of each stiffener is first welded, without introducingfiller metal, by heating the bearing lip 18 and the associated mainsurface 12A of the plate from inside the conduit 16. This heating andmelting of the bearing lip is performed, for example, by TIG welding orby plasma welding and its variants, in other words by inert gasnonconsumable-electrode arc welding without filler metal.

For this purpose, according to a first method of implementation of theinvention shown in FIG. 6, the section of member to be welded isarranged such that the bisector of the general angle defined between themain surface 12A and the bevel 24 extends more or less vertically.

A self-propelled trolley 60 fitted with an observation camera andguidance means is introduced into the conduit 17. The trolley 60 isfitted with rollers 64, which bear on the internal lateral surface 13Aof the stiffener and on the plate main surface 12A.

Moreover, the trolley carries a nonconsumable-electrode weldingelectrode 66 enclosed in a tube 68 supplying inert gas such as weldinggas (argon or helium) mixtures. The electrode 26 is held at distancefrom the bearing lip 18 such that an electric arc 70 is created betweenthe electrode and the corner defined by the bearing lip 18 and the plate12, a potential difference being applied between the section of memberto be welded and the electrode 26.

Preferably, the arc is created in the immediate vicinity of theintersection of the main surface 12A and the bearing lip 18, this areabeing shown by the arrow F in FIG. 4.

The temperature rise in the bearing lip 18 causes its constituent metalto melt, at least in the parts of it featuring the internal lateralprojection 28 and in contact with the surface 12A.

The lip 18 therefore plays the part of filler metal and, under theaction of heat, enables satisfactory weld penetration to be obtainedbetween the stiffener 13 and the plate 12 in the bearing lip region.

Working from inside the conduit 17, welding without filler metal isperformed throughout the length of the bearing lip 18 and, inparticular, throughout the length of the stiffener 13 if the bearing lipextends over its full length.

In the alternative method of implementation illustrated in FIG. 7, themember is positioned such that the plate 12 lies with its main surface12A generally horizontal. The self-propelled trolley 60 rolls along thissurface on wheels 72 and positions itself with respect to the stiffener13 by means of a lateral guide roller 74. In this implementation methodalso, the self-propelled trolley moves the electrode 66 along the lengthof the bearing lip 18 to create an electric arc 70, ensuring melting ofthe internal part of the bearing lip.

It is noted that in the two alternative methods of implementation of theprocess, the internal lateral projection 28, associated with initialcurvature of the concave section 30, gives a weld 78 ensuring connectionof the main surface 12A and the bevel 24 that features a curved externalprofile 79 generally linked tangentially, on the one hand, to the bevel24 and, on the other hand, to the main surface 12A.

After internal welding without filler metal, the clamping flanges 40 areremoved and the space defined between the bearing surface 12A and theexternal bevel 20 is filled with an external weld bead 80, as shown inFIG. 8. The weld 78 is used as a backing for the first external weldpasses.

This weld bead is obtained by automatic flux welding using a processknown in itself.

The filler metal used for forming the weld bead 80 is such that the bead80 fills completely and accurately the space defined between the mainsurface 12A and the external bevel 20, the free surface 82 of the weldextending in the projected external general surface 13B of thestiffener.

When laying down the external weld bead 80, the external part of thebearing lip melts due to heating and is thereby welded to the plate 12,as shown in FIG. 8.

Depending on the thickness of the stiffener, the position of the bearinglip 18 is determined by calculating the forces to be applied to theweld. The width of the weld bead 80 is determined from these forces andthe bearing lip position is defined such that the width of the weld bead80 is equal to the width of weld required to obtain the necessarystructural characteristics.

It can be seen that the fatigue strength of the weld is high using sucha welding process. Furthermore, implementation of the welding process isrelatively easy because no welding with filler metal is performed frominside the conduit 17 and filler metal is only introduced from outside.

The presence of the bearing lip 18 makes welding without filler metalpossible from the inside of the conduit.

In an alternative method of implementation to ensure welding withoutinternal filler metal, the self-propelled trolley 60 does not move alongthe length of the section to be welded inside the space delimited by thestiffener but, on the contrary, outside the space 17, depending on thelength of the section. The electrode 66 performing welding is carried atthe end of an arm built into the self-propelled trolley and insertedinto the space 17 from an open end of this space.

The welding process can be implemented in all positions and, inparticular, with the weld to be executed in a vertical configuration.

1. A fabrication process for a section (11) of an offshore oilproduction platform support leg (2) member (10), the member (10)comprising a main plate (12) incorporating, on at least one longitudinaledge, teeth (14) forming a rack and at least one stiffener ofsemicylindrical shape welded to a main surface (12A) of the main plate(12) along two longitudinal edges (16), the stiffener (13) and the plate(12) delimiting a conduit (17), wherein it includes the following stagesapplied to at least one part of the length of the member (10): a)machining of each longitudinal edge (16) of the stiffener (13) to form,along the longitudinal edge (16), a lip (18) for bearing on the mainsurface (12A) of the main plate (12) by forming, along the longitudinaledge (16), an external bevel (20) on the side opposite the conduit (17);b) applying a sole (22) of the bearing lip (18) to the main surface(12A) of the plate (12); c) heating the bearing lip (18) to ensure itswelding to the plate (12) and to form a bearing weld (78); and d) withfiller metal, forming a weld bead (80) from outside the conduit (17)within the space defined between the bearing weld (78), the externalbevel (20) and the main surface (12A) of the main plate (12).
 2. Theprocess as claimed in claim 1, wherein the heat supply causing heatingof the bearing lip (18) for the purpose of welding it to the plate (12),is ensured from inside the conduit (17).
 3. The process as claimed inclaim 1, wherein heating of the bearing lip (18) to ensure its weldingto the plate (12) is performed without introducing filler metal.
 4. Theprocess as claimed in claim 1, wherein heating of the bearing lip (18)to ensure its welding to the plate (12) is performed using an inert gasnonconsumable-electrode arc welding method.
 5. The process as claimed inclaim 1, wherein it includes a stage, applied to at least one part ofthe length of the member (10), involving machining of each longitudinaledge (16) of the stiffener (13) to form an internal bevel (24) on thisedge, on the side of the bearing lip (18) facing the conduit (17). 6.The process as claimed in claim 1, wherein the weld bead (80) formedfrom outside the conduit (17) fills completely the space defined betweenthe main surface (12A) of the plate, the external bevel (20) and thebearing lip (18).
 7. The process as claimed in claim 1, wherein thebearing lip (18) is incorporated within the half of the stiffener (13)thickness (e) situated on the internal conduit (17) side.
 8. The processas claimed in claim 1, wherein the distance (d) separating the stiffenerinternal lateral surface (13A), defined on the conduit (17) side, fromthe bearing lip (18) is between 20 and 40% of the stiffener (13)thickness (e).
 9. The process as claimed in claim 1, wherein the bearinglip (18) features an internal lateral projection (28) on the conduit(17) side near to its sole (22).
 10. The process as claimed in claim 9,wherein the minimum height (T) of the projection (28) is between 0 and 8mm.
 11. The process as claimed in claim 6, wherein the bearing lip (18)features an internal lateral projection (28) on the conduit (17) sidenear to its sole (22) and wherein a hollow profile (30) is definedbetween the projection (28) and the internal bevel (24).
 12. The processas claimed in claim 1, wherein the height (h) of the bearing lip (18)measured at the base of the external bevel (20) is between 6 and 12 mm.13. The process as claimed in claim 1, wherein the width of the sole(22) is between 2 and 15 mm.